A small-scale fuel cell system capable of carrying out high-efficiency electric power generation has been developed as a distributed power generating system capable of realizing high energy use efficiency, since it is easy to configure a system which utilizes heat energy generated when carrying out an electric power generating operation.
When carrying out the electric power generating operation, a hydrogen-containing gas containing hydrogen and an oxygen-containing gas containing oxygen are supplied to a fuel cell stack (hereinafter simply referred to as “fuel cell”) provided as a main body of an electric power generating portion of the fuel cell system. Then, in the fuel cell, a predetermined electrochemical reaction proceeds by using the hydrogen contained in the supplied hydrogen-containing gas and the oxygen contained in the supplied oxygen-containing gas. By the progress of the predetermined electrochemical reaction, chemical energies of the hydrogen and the oxygen are directly converted into electric energy in the fuel cell. With this, the fuel cell system outputs electric power to a load.
Here, a system for supplying the hydrogen-containing gas necessary during the electric power generating operation of the fuel cell system is not usually arranged as an infrastructure. Therefore, in a conventional fuel cell system, a reformer is typically disposed with the fuel cell. The reformer uses a material gas, such as city gas or LPG, obtained from, for example, an existing fossil material infrastructure, and steam generated by a water evaporator to progress a steam-reforming reaction at a temperature of 600° C. to 700° C., thereby generating the hydrogen-containing gas. Generally, the hydrogen-containing gas obtained by the steam-reforming reaction contains a large amount of carbon monoxide and carbon dioxide which are derived from the material gas. Therefore, in the conventional fuel cell system, a shift converter and a selective oxidizer are typically disposed with the fuel cell and the reformer in order to reduce the concentration of the carbon monoxide contained in the hydrogen-containing gas generated in the reformer. The shift converter decreases the temperature of the hydrogen-containing gas and progress a water gas shift reaction at a temperature of 200° C. to 350° C. to reduce the concentration of the carbon monoxide, and the selective oxidizer progresses a selective oxidation reaction at a temperature of 100° C. to 150° C. to further reduce the concentration of the carbon monoxide. In the conventional fuel cell system, the reformer, the shift converter, and the selective oxidizer constitute a hydrogen generator. Each of the reformer, the shift converter, and the selective oxidizer is provided with a catalyst suitable for the steam-reforming reaction, the water gas shift reaction, or the selective oxidation reaction to progress the corresponding chemical reaction. For example, the reformer is provided with a Ru catalyst or a Ni catalyst, the shift converter is provided with a Cu—Zn catalyst or a precious metal based catalyst, and the selective oxidizer is provided with the Ru catalyst or the like.
In the hydrogen generator having the above configuration, generally, the temperatures of respective reactors need to be maintained at optimal temperatures to appropriately progress the chemical reactions of the respective reactors. In addition, in the hydrogen generator having the above configuration, an important object is to effectively utilize the heat energy necessary to maintain the temperatures of the respective reactors at the optimal temperatures.
Here, proposed is a hydrogen generator in which a cylindrical reformer, a water evaporator, a shift converter, and a selective oxidizer are concentrically disposed around the heater (see Patent Document 1 for example).
In the case of the configuration of the hydrogen generator described in Patent Document 1, generally, when supplying to the reformer the steam generated in the water evaporator disposed on an outer peripheral side of the reformer, the flow of the steam supplied from the water evaporator is changed from an axial direction of the water evaporator to a circumferential direction thereof. Because of this, the configuration of a steam passage becomes complex. Therefore, for example, a mixture gas supplying pipe which connects the steam passage and an entrance of the reformer needs to extend in a radial direction, and a connection portion where the mixture gas supplying pipe and the reformer extending in the axial direction are connected to each other needs to be subjected to a piping installation, such as welding. The piping installation of the steam passage may increase the cost of the hydrogen generator and deteriorate the durability performance of the hydrogen generator.
Here, proposed is a hydrogen generator in which a tubular water evaporator and a tubular reformer are arranged to be lined up in the same axial direction (see Patent Document 2 for example).    Patent Document 1: Japanese Laid-Open Patent Application Publication 2002-187705    Patent Document 2: Japanese Laid-Open Patent Application Publication 2005-225684